This invention relates to a method and system for the removal of solids from a liquid containing suspended solids by flowing the liquid through a filter bed of polyethylene particles and periodically cleaning the polyethylene particles.
Filtration has long been a major method of removing suspended solids from liquid streams. Due to governmental regulations and controls, and in an effort to maintain the purity of the streams and rivers, waste water treatment is often necessary before the waste water can be discharged from industrial applications. Suspended solids must be removed. In the past the most widely used filters have been sand filters or mixed media filters. However, sand and mixed media filters are generally effective in removing solids only to a limited extent. The reason is that these sand and mixed media filters generally will clog in situations in which the solids are highly concentrated. Furthermore, when the filter bed begins to clog, there is an extremely high pressure drop across the filter bed.
Some filtering systems have used a filter bag, cartridge, or filter aid, all of which require filter media replacement. The obvious disadvantage of these type of systems is that when the filter becomes clogged or used up, an operator must attend to the removal and replacement of the filter media. Furthermore, attempts to provide a backwash to cleanse heavier than liquid filters in these systems generally proved to be inefficient.
One application which utilizes polyethylene beads that are floated in the fluid is disclosed in U.S. Pat. No. 3,424,674. This patent discloses a polyethylene filter bed in which the filter material is constantly circulated, cleaned and then returned to the filter bed. The problem with this invention is that it requires a fairly complex mechanism to remove the polyethylene particles from the filter bed for cleaning and then provide a means for returning the cleaned particles back to the bed. In another embodiment, a separate paddle mechanism is used to agitate the filter bed in order to provide the flow of filter particles through the cleaning means. The '674 patent teaches away from the backwashing technique to clean the filter bed. The reasons given are that backwashing requires substantial volumes of clean fluid, that the entire filter bed is disrupted and filtration must be suspended during the backwash interval until the filter material once again has resettled into a filter bed.
Another patent directed toa filtration system is U.S. Pat. No. 4,162,216. This disclosure illustrates the use of a discontinuous polyurethane particle filter bed. The liquid containing suspended solids is flowed through the filter bed which traps the solids. A regeneration liqid is added to the filter vessel and the filter bed is mechanically mixed. The liquid is discharged from the vessel for further cleaning. The problem with the use of polyurethane particles is that they sink to the bottom of the vessel and do not provide adequate filtering means. Furthermore, they cannot easily be cleaned by the mechanical mixing as described in the '216 patent. The shortcomings are made more apparent by the fact that the inventor states that it may be necessary to add a coagulant material in order to enhance the efficiency of the filter bed. Another problem with using polyurethane foam as a filtering material, is that during backwashing a high flow velocity through the filter bed is required to wash the solids from the polyurethane foam. This often causes particles of the polyurethane foam out of the filter bed or otherwise damages the filter bed. This further highlights the problem of utilizing polyurethane foam as a filtering material.
Accordingly, is it an object of the present invention to provide an improved method and system for removing suspended solids from liquids. Related to this object is the object of providing a bed of polyethylene particles which serves as a floating filter media.
Another object is to provide a system that backwashes the accumulated solids from the filter bed by the use of non-filtered liquid and collects the accumulated solids into a storage tank where they can be further processed and removed. An advantage of the inventive system is that the backwashing is accomplished with on-stream liquid and therefore does not introduce fresh or filtered fluid into the system.
Yet another object is to provide a filtering system which provides for backwashing solids from the filter bed without the use of mechanical paddles in the filtering vessel.
Still another object is to provide a filtering system which does not require periodic replacement of the filtering media, nor require filter bags, cartridges, or other disposable components.
Another object is to provide a filtering system that can be made of food grade components and resistant to most chemicals.
Other objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment and appended claims.